1. Field of the Invention
This invention relates to the separation of normal paraffins from hydrocarbon vapor mixtures thereof with non-normal hydrocarbons. More particularly, it relates to the separation of said normal paraffins present in high concentrations in petroleum naphthas.
2. Description of the Prior Art
The separation of normal paraffins from a hydrocarbon vapor feed stream has been developed in the art, as indicated by the Avery patent, U.S. No. 3,422,005. With respect to gas oil and kerosene feedstocks in particular, the patent discloses the isobaric steps of (1) adsorption, i.e., selective adsorption of normal paraffins; (2) cocurrent purge with n-hexane to sweep out void space vapor containing a high concentration of non-adsorbed components, i.e., non-normal hydrocarbons, from the upper or effluent end of the bed, and (3) countercurrent purge with n-hexane to desorb normal hydrocarbons for discharge from the bottom or feed end of the bed. As will be appreciated by those skilled in the art, various changes or modifications in such processing techniques may be necessary or desirable when other feedstocks are to be treated for such separation of normal paraffins and non-normal paraffins. In the treatment of petroleum naphthas, both light and heavy, a four-step cyclic process variation is commonly employed and includes (1) cocurrent purge/adsorption, i.e., selective adsorption of normal paraffins from the feed gas passed to the bottom or feed end of the bed, with unadsorbed non-normal paraffins displacing residual purge gas remaining from the previous cycle from the top or effluent end of the bed, said step being sometimes referred to herein as the A-1 step; (2) cocurrent feed/adsorption, wherein additional quantities of the feed gas are mixed with the purge effluent from the next succeeding countercurrent purge step and are passed to the bottom of the bed, thereby advancing the adsorption front of adsorbed normal paraffins toward the top of the bed, thus displacing non-normal paraffins from the top of the bed for recovery as a co-product stream, said step being sometimes referred to herein as the A-2 step; (3) countercurrent purge, in which a stripping gas is introduced to the top of the bed and a purge effluent comprising said stripping gas, residual feed components, residual unadsorbed non-normal paraffins, and some desorbed normal paraffins are withdrawn from the bottom of the bed and are recycled for mixture with feed gas and introduction to the bottom of another bed, said countercurrent purge step being sometimes referred to herein as the D-1 step; and (4) countercurrent displacement, in which said stripping gas is introduced to the top of the bed and a normal paraffins-stripping gas product stream is withdrawn from the bottom of the bed, said countercurrent displacement step being sometimes referred to herein as the D-2 step. It will be appreciated that said cyclic process is commonly employed in multi-bed systems, typically containing at least four beds, in which said processing steps are carried out, on a cyclic basis, in each bed in overlapping processing sequence.
In the practice of this four-step process, a slipstream of the hydrocarbon feed gas is used as the feed gas for the A-1 step. The remaining hydrocarbon feed gas is mixed with the D-1 effluent in a mix drum to form A-2 step feed gas. The mix drum facilitates the providing of an even A-2 feed composition during those periods in the cycle in which two beds are simultaneously on the A-2 step. The processing cycle of the prior art, as applied to a five-bed cycle, is illustrated in table I.
It will be seen that the slipstream of the feed gas used for A-1 feed to the system is continuous, with such A-1 feed commencing in bed 2 upon termination of the A-1 step in bed 1, commencing in bed 3 upon termination of said step in bed 2, and the like. Similarly, the D-1 step is carried out in one bed of the system at any given time on a continuous basis. Thus, the termination of the D-1 step in bed 3 is accompanied by the commencing of said step in bed 4, such termination in bed 4 is accompanied by its commencement in bed 5, and the like. It will also be seen that the A-2 and D-2 steps are carried out in such overlapping sequence that, alternately, one bed or two beds simultaneously are on said steps at any point in the overall processing cycle. Flow controllers applied to the A-1 slipstream feed gas and to the D-1 purge gas are thus continuously operating to control the required amount of flow through their associated control valves. No special bypass or controller hook-up is needed to protect said valves from a no-flow condition as part of the processing cycle in said system.
In the carrying out of such petroleum naphthas separation operations, further described in the Holcombe Patent, U.S. No. 4,176,053, it has been found necessary to utilize a five-bed system for the treatment of high normal paraffin concentration feedstocks. Such feedstocks are typically those for which about 80% or more of the total feed gas would be needed as A-1 feed gas in a four-bed system. During the A-1 step, normal paraffins are selectively adsorbed by the bed, with the remaining, unadsorbed non-normals serving to push the stripping gas remaining from the previous D-2 step from the top of the bed. If the normals concentration of the hydrocarbon feedstock is high, a large portion of the total feedstock will be needed to remove the required amount of stripping gas from the bed during said A-1 adsorption step. In a four-bed system, it is possible that, at high normals concentration, essentially all of the feedstock to the system will be needed for the A-1 step, leaving essentially none of the original feedstock available for mixing with the countercurrent purge, i.e., D-1 step, effluent and for use in the A-2 step. By utilizing a five-bed system containing the same total amount of adsorbent material, the A-1 step can be carried out at a lower feed rate, because it is carried out on a continuous basis throughout the cycle. The amount of stripping gas that needs to be removed from the bed during the A-1 step, therefore, is removed over a longer period of time relative to a corresponding four-bed system with a greater proportion of the total cycle time being available for purging.
While the conventional five-bed system serves to overcome the disadvantages encountered in attempting to treat high normal paraffin-containing feedstocks in a corresponding four-bed system, it would nevertheless be desirable to employ four-bed systems in such an application. A significant savings in investment could thus be realized by the use of one less adsorbent bed, with a corresponding reduction in associated manifolding, and a total of six related Remote Operated Valves (ROV's) could be eliminated if a suitable four-bed system could be effectively employed in this application.
It is an object of the invention, therefore, to provide an improved process and system for the separation of normal paraffins from vapor mixtures thereof with non-normal paraffins.
It is another object of the invention to provide an improved process and system for such separation operations suitable for the treatment of high normal paraffin concentration feedstocks.
It is a further object of the invention to provide a four-bed process and system for separation of normal paraffins contained at high concentrations in vapor phase mixtures thereof with non-normal paraffins.
With these and other objects in mind, the invention is hereinafter further described in detail, the novel features thereof being particularly pointed out in the appended claims.